1. Field of the Invention
This application relates to devices and methods for recharging backup power supplies used in telecommunications systems, and more particularly to such devices and methods with thermal runaway control.
2. Description of the Prior Art
Electrical energy cost for the operation of sophisticated telecommunications systems has increased disproportionately to the other plant operation costs. The energy cost increase includes significant contributions other than merely the increases in power companies rates. Part of the reason is a change in the power consumption characteristics of newer telecommunication equipment.
Power consumption for older telecommunications processing equipment, for the most part, followed telephone traffic demand. High volumes of telephone traffic resulted in high volumes of power consumption. This type of energy demand resulted in small numbers of telephone power generating units connected during low demand times and large numbers of telephone power generating units connected during high demand times. Power control was accomplished with elaborate circuitry, and resulted in older telephone power plants being very energy efficient.
On the other hand, power consumption for newer sophisticated telecommunications processing equipment is static, i.e., relatively constant irrespective of telephone traffic demand. Thus, the associated telephone power generating units are all connected and share the telephone power consumption demand. Telephone power generating units are now, for the most part, state of the art, controlled ferroresonant, SCR (silicon controlled rectifier) and Switch Mode rectifiers. When these types of rectifiers are used in a method of random output with all units connected, and without proportional load sharing and/or control, as is done in some present telephone power plants, energy inefficiencies result.
Experience has shown that electrical power consumption in the telecommunications industry is not only large, but is increasing. Thus, seemingly small positive efficiency changes have the potential to yield large energy cost savings. For example, a 48 volt, 500 ampere telephone power plant consumes approximately 25,000 watts of electrical energy: EQU volts.times.(amps/efficiency).times.power factor=watts input
This amount equates to 25 kwhr (watts/1000=kwhr) every hour, every day, and every year. At a typical cost of eight cents per kwhr, this equates to an energy cost of $17,520.00 per year: EQU kwhr.times.cost/kwhr.times.24(hrs/day).times.365(days/year)=energy cost/yr
Every positive energy efficiency percentage point change would result in an ongoing $175.00 per year energy cost savings. Of course, as telecommunications demand increase, there will be a subsequent increase in telephone power plant sizes and numbers. Use of an energy management system with this growth of power plants can yield large energy cost savings.
Energy control systems are already in existence, but are usually pan of a very complex and expensive circuit design. Such existing circuits are primarily used for remote monitoring and diagnostics of telephone power systems, and usually do not provide thermal runaway control.
Thermal runaway is a detrimental and often dangerous condition that can occur in certain types of newly developed sealed, valve-regulated lead acid batteries that are used for backup power in telecommunications systems. This condition can occur when the ambient temperature is at 110.degree. F. or higher and/or excessive recharge current is used. In either case, a degradation in open circuit cell voltage is caused by high internal temperatures created as a result of either high ambient temperatures or increased I.sup.2 R heat from excessive recharge current. In addition, a decrease in open circuit cell voltage results in a larger than normal difference of potential between open circuit cell voltage and float voltage. This abnormally large potential difference produces more than normal float current, which, in turn, produces excessive hydrogen gas, which has proven to be explosive when mixed with air in concentrations of 3% or more by volume. In addition, the excessive float current also produces increased I.sup.2 R heat which further degrades open circuit voltage which produces yet additional current. The snowballing effect results in thermal runaway.
Excessive recharge current, which may result in thermal runaway, is normally considered to be current in excess of one quarter of the ampere-hour rating of the lead acid battery. Due to engineering practices for sizing batteries and rectifiers based on projected busy hour loads and future growth, it is quite common to have excessive recharge current capacity in telephone power plants. Older technology flooded lead acid batteries are very forgiving. Such batteries have more liquid electrolyte than do sealed batteries. Thus, I.sup.2 R heat is conducted out of the cell by the electrolyte, allowing the batteries to be quite tolerant of excessive I.sup.2 R heat as a result of excess current. However, the electrolyte in the new valve-regulated batteries is suspended in fiberglass, and does not allow the batteries to tolerate heat as well as flooded technology batteries.
Some energy management systems exist that attempt to control thermal runaway by monitoring temperature and battery string voltage, and from this information, control current to the batteries by reducing rectifier output voltage. While effective, these techniques have led to complicated and expensive energy management systems.
It would thus be desirable to have a simple, inexpensive energy management system for telecommunications equipment that provides local energy management along with thermal runaway control.